1. Field of the Invention
The present invention relates to a manufacturing method of a semiconductor device suitable for a ferroelectric memory.
2. Description of the Related Art
A flash memory and a ferroelectric memory are known, as a nonvolatile memory that can still store information after the power supply is turned off.
In the flash memory, a floating gate is buried in a gate insulating film of an insulated-gate field effect transistor (IGFET), and information is stored by accumulating electric charges expressing memory information in the floating gate. A tunnel current that passes through the insulating film has to be passed to write and erase the information, and thus comparatively high voltage is needed.
On the other hand, the ferroelectric memory stores information by using a hysteresis characteristic of ferroelectric substance. A ferroelectric capacitor having a ferroelectric film as a capacitor dielectric between a pair of electrodes generates a polarization in response to an applied voltage between the electrodes and has spontaneous polarization even after the applied voltage is removed. If polarity of the applied voltage is inverted, the polarity of the spontaneous polarization is also inverted. The information can be read by detecting the spontaneous polarization. The ferroelectric memory can be driven by the low voltage as compared with the flash memory, and thus can execute high-speed writing with lower power consumption.
The ferroelectric film of the ferroelectric capacitor is formed of a PZT material such as lead-zirconate-titanate (PZT) and La-doped PZT (PLZT), a Bi-layer structure compound such as SrBi2Ta2O9 (SBT, Y1) and SrBi2(Ta, Nb)2O9(SBTN, YZ), or the like.
Conventionally, as the method of forming the ferroelectric film, a sol-gel method or a sputtering method is used. By these film forming methods, the ferroelectric film of an amorphous phase is formed on the lower electrode, and thereafter, the ferroelectric film is crystallized to be a crystal of the perovskite structure by thermal treatment.
Since the ferroelectric film is crystallized in an oxygen atmosphere, an capacitor electrode is formed of precious metal such as Pt, and IrO2, SrRuO3, La0.5Sr0.5CoO3 or the like that have conductivity even after oxidized. As for the upper electrode, an interlayer insulating film in a multilayer wiring structure is formed in a reducing atmosphere, and therefore when an upper electrode is formed of precious metal such as Pt and Ir, hydrogen in the reducing atmosphere enters the Pt film or the Ir film and is activated by the catalytic action these metals have. As a result, the ferroelectric film in the ferroelectric capacitor is reduced by the activated hydrogen. When the ferroelectric film is reduced, the operating characteristics of the ferroelectric capacitor are sharply deteriorated. Therefore, conductive oxide which does not have the catalytic action is generally used as raw material of the upper electrode.
For example, Patent Document 1 (Japanese Patent Application Laid-open No. 2002-324894) describes that the upper electrode formed on the ferroelectric film is constituted of a first conductive oxide film and a second conductive oxide film, and the second conductive oxide film is formed to have a composition closer to a stoichiometric composition than the first conductive oxide film.
By making a composition of the first conductive oxide film in contact with the ferroelectric film of the ferroelectric capacitor a non-stoichiometric composition, Pb is diffused into the first conductive oxide film from the ferroelectric film. With this, an interface between the ferroelectric film and the upper electrode is planarized. As a result, a value of effective voltage applied to the ferroelectric film when a voltage is applied to the ferroelectric capacitor becomes larger, and the capacitor characteristics are improved.
However, when such a conductive oxide film of the non-stoichiometric composition is exposed to an atmosphere including hydrogen, a metal component in the film activates hydrogen, and the activated hydrogen deteriorates the ferroelectric film.
Thus, Patent Document 1 discloses a method for blocking entry of a reduction atmosphere into the first conductive oxide film by forming the second conductive oxide film having the stoichiometric composition or the composition closer to the stoichiometric composition on the first conductive oxide film.
Patent Document 2 (Japanese Patent Application Laid-open No. 2002-246564) describes an annealing method in a case in which the upper electrode formed on the ferroelectric film is made of a conductive oxide film. In this method, after the ferroelectric film (PZT film) of the amorphous phase is formed, it is crystallized to the perovskite structure by a first rapid thermal annealing (RTA). Next, after a conductive oxide film is formed as the upper electrode, crystallization of the PZT is completed by a second RTA. The capacitor characteristics are improved by simultaneously annealing the PZT film and the conductive oxide film (upper electrode film).
However, it describes that when a process step of simultaneously annealing the PZT film and the conductive oxide film (upper electrode film) by RTA is performed in a mixed gas containing oxygen of high partial pressure, foreign matters are formed on the upper electrode film. It is possible that these foreign matters interfere with subsequent process steps, and therefore it is necessary to prevent the foreign matters from being formed. Japanese Patent Application Laid-open No. 2002-246564 describes that when a process step of annealing the PZT and the upper electric layer together by RTA after film forming is carried out in a mixed gas containing about 1% of oxygen in an inert atmosphere such as an argon gas, generation of the foreign matter is inhibited.
However, even with adoption of any method of them, a sufficient quantity of switching electric charge to be demanded hereinafter cannot be obtained.